The agency also urged state authorities across Europe to do more to assist and protect refugees and migrants - and expressed concern about abuses by criminal gangs, including kidnappings, physical abuse, threats and extortion.

"We urge European states to enhance their efforts to tackle these criminal networks and ensure the safety of refugees and migrants," Pouilly said.

In Serbia, where temperatures have been below zero for the past 10 days, about 1,200 men are living rough in inadequate informal sites in Belgrade city centre - including up to 300 unaccompanied or separated boys.

UNHCR has worked with the authorities and other partners to provide heaters, thermal blankets, winter clothes and footwear to people in Belgrade.